An adverb meaning last year, a year ago. Paul uses it to remind the Corinthians of their early enthusiasm for the collection for Jerusalem's saints.
Paul reminds the Corinthians: 'Last year you were the first not only to give but also to have the desire to do so' (2 Corinthians 8:10). Initial enthusiasm can fade. Finishing what we start is as important as starting it. Desire without completion is incomplete obedience.